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Thread: How does the type of a star affect an orbiting planet's Bond albedo?

  1. #1
    Join Date
    Jun 2008

    Question How does the type of a star affect an orbiting planet's Bond albedo?

    According to this Wikipedia article on gas giant classes:

    Water clouds are more reflective than ammonia clouds, and the predicted Bond albedo of a class II planet around a Sun-like star is 0.81. Even though the clouds on such a planet would be similar to those of Earth, the atmosphere would still consist mainly of hydrogen and hydrogen-rich molecules such as methane.
    The bolded part is what I'm interested in: it seems to suggest that a different star would result in a different Bond albedo. For example, Earth has a Bond albedo of around 0.306. Would it still have the same albedo if the Sun was, say, a red dwarf? If not, would it be lower or higher?

    I've looked around but haven't managed to find much on this.
    “Of all the sciences cultivated by mankind, Astronomy is acknowledged to be, and undoubtedly is, the most sublime, the most interesting, and the most useful. For, by knowledge derived from this science, not only the bulk of the Earth is discovered, but our very faculties are enlarged with the grandeur of the ideas it conveys, our minds exalted above their low contracted prejudices.” - James Ferguson

  2. #2
    Join Date
    Jul 2005
    Bond albedo is the fraction of incident EM power reflected. Some parts of the EM spectrum will be absorbed better than others, making a planet less reflective at those wavelengths. If the spectrum of the parent star is rich in EM wavelengths that the planet absorbs efficiently, it will have a low Bond albedo. Put the same planet under a star that radiates primarily in wavelengths the planet reflects, and it will have a higher Bond albedo.
    As a toy example, illuminate a red planet with monochromatic red light, and its Bond albedo is high; illuminate it with monochromatic green light, and its Bond albedo is low.
    To work out the Bond albedo of the Earth under another illuminant, you'd need to look at the wavelength dependency of the EM radiation it reflects.

    Grant Hutchison

  3. #3
    Join Date
    Jun 2003
    Water clouds would reflect the light from a red dwarf well, so the Bond albedo of a planet with water clouds would not be affected very much. But water ice does not reflect reddish light quite as well, so the amount of light reflected from a red dwarf by an icy planet would be reduced.

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